1. Field of the Invention
The invention relates to the production of high octane number motor fuel via a combination of alkylation and fluidized catalytic cracking, or FCC. In particular, this invention relates to the alkylation of isobutane with a C.sub.3 or C.sub.4 mono-olefin to form an alkylate readily separable into a high octane and a low octane alkylate fraction. The low octane alkylate fraction is sent to an FCC unit to produce more olefins. The olefins produced are recycled to the alkylation process, resulting in an increased production of high octane number alkylate.
2. Description of the Prior Art
Production of C.sub.6 to C.sub.10 highly branched paraffins having high octane numbers is of great importance to the refinery industry. High octane number fuels are needed for high performance automotive engines and aviation engines. Higher octane fuels are needed because of the worldwide trend toward eliminating or reducing the amount of lead anti-knock additives permitted in gasoline. Refiners have for years relied on the addition of lead antiknock additives to increase product octane number to that required by modern engines. The prospect of phasing the lead out of gasoline leaves refiners with the dilemma of producing higher octane gasolines.
A common source of high octane fuel is catalytic alkylation of low boiling isoparaffins, e.g., isobutane, with mono-olefins such as propylene, butylenes, amylenes, and mixtures thereof. The typical commercial alkylation processes of today usually alkylate isobutane with butylenes and/or propylene. These commercial processes typically produce a motor fuel alkylate having a research clear octane rating of about 93 to 95.
It is well recognized in the prior art that the components present in a typical motor fuel alkylate constitute a diverse mixture of both high octane and low octane C.sub.5 to C.sub.10 isomers. The prior art also recognizes that the only portion of the alkylate product having a low octane number which can be readily separated from the total alkylate product is the high boiling portion of the alkylate, commonly referred to in the art as alkylate bottoms. In the typical alkylate produced by the catalytic alkylation of isobutane and isopentane with a typical olefin mixture of propylene, butylenes, and amylenes, the alkylate has a 50% volumetric distillation temperature at atmospheric pressure of about 90.degree. to 115.degree. C, and a 95% temperature of about 150.degree. to 175.degree. C, and infrequently as high as 200.degree. C. The exact initial boiling point varies with the amount of light ends, e.g., butane, present in the fuel. The amount of light ends desired will vary with the seasons.
The art has recognized that a problem exists with conventional motor fuel alkylation processes. Most of this recognition has been that one fraction of the alkylate product possessed a low octane number, and in general, the solution comtemplated was either reforming of this low octane fraction or charging it to an isomerization zone.
In U.S. Pat. No. 3,502,569 (Class 208-49), the teachings of which are incorporated by reference, the patentee recognized that in a typical alkylate fraction there was a certain fraction of the alkylate which contained a relatively high dimethylhexane content. The dimethylhexanes are very low in octane and were dragging down the overall octane number of the alkylate product. The solution proposed by this patentee was to charge at least a portion of the dimethylhexane-rich fraction to a catalytic reforming zone, and ultimately blend in the reformate with the remaining, and higher octane, fraction of alkylate. Although the solution proposed in this patent would provide a significant increase in product octane, there are many refineries in which it would be impossible to practice. Thus, if a refiner had limited capacity in a catalytic reforming process, he could not tolerate the incremental charge stock from the alkylation unit effluent. Similarly, there is concern over the aromatic content of gasolines. Refiners are facing the spectre of producing a gasoline with a certain maximum of allowable aromatic content. Sending an increasing proportion of the charge stock through a catalytic reformer would increase the overall aromatic content of a refiner's gasoline pool. Thus, refiners may not have the equipment available to practice the invention in U.S. Pat. No. 3,502,569, and it is also possible that product specifications will deny refiners the option provided in that patent.
An alternative solution to this problem, i.e., of upgrading the relatively low octane number fraction of an alkylate containing a high dimethylhexane content, was disclosed in U.S. Pat. No. 3,686,354 (Class 260/683.43), the teachings of which are incorporated by reference. This reference is the closest prior art known to applicant. In this patent, the dimethylhexane fraction of the alkylate is upgraded by charging it to a disproportionation and/or transalkylation reaction zone. It is believed that the patentee was trying to convert a substantial portion of his dimethylhexane production to trimethylpentane, which has a substantially higher octane number. Unfortunately, for all catalysts known to the applicant, the equilibrium is very unfavorable for this reaction. The ratio of dimethylhexane to trimethylpentane will always be about 9 to 1. It is theoretically possible, by separation and recycle of reactants charged to the transalkylation and disproportionation zone, to increase the yield of trimethylpentane, but the expense will usually not justify such extreme methods.
Because of the deficiencies, or perhaps unrecognized problems, inherent in the prior art methods of improving the product octane number of an alkylate, I studied the work that others had done in an attempt to find a better way to improve product octane.
I discovered that there was a much more efficient way to convert the low octane number fraction of a product alkylate into something which would increase the yield of high octane, paraffinic, motor fuel. The invention is the realization that charging a relatively low octane fraction to a catalytic cracking unit results in the almost stoichiometric conversion of this low octane fraction to olefins and other light and useful materials which may be used in producing alkylate.